The present invention relates to a mass spectrometer, and more specifically to a multi-turn time-of-flight mass spectrometer or a Fourier-transformation mass spectrometer including an ion optical system in which ions are made to fly repeatedly along a closed loop orbit.
In a time-of-flight mass spectrometer (TOF-MS), the mass of an ion is generally calculated from the time of flight which is obtained by measuring a period of time required for the ion to fly at a fixed distance, on the basis of the fact that an ion accelerated by a fixed energy has a flight speed corresponding to the mass of the ion. Accordingly, elongating the flight distance is particularly effective in enhancing the mass resolution. However, elongation of a flight distance on a straight line requires unavoidable enlargement of the device, which is not practical, so that a mass spectrometer called a multi-turn time-of-flight mass spectrometer has been developed in order to elongate a flight distance.
In such a multi-turn time-of-flight mass spectrometer as disclosed in Patent Document 1 for example, the flight distance is effectively elongated by forming a figure-eight (“8”) shaped closed loop orbit using two to four of the sector-formed electric fields and causing ions to repeatedly fly along this loop orbit multiple times. In a multi-turn time-of-flight mass spectrometer disclosed in Patent Document 2, the flight distance is effectively elongated by forming a quasi-polygon shaped closed loop orbit using multiple sector-formed electric fields and causing ions to repeatedly fly along this loop orbit multiple times. This construction can make the flight distance free from limitation due to the entire device size and mass resolution improve as the number of turns increases.
In the multi-turn time-of-fight mass spectrometer as stated earlier, an ion source is placed outside a loop orbit. Departed ions from this ion source are introduced into the loop orbit and begin flying along it. An ion detector is placed outside the loop orbit, and ions which have turned around along the loop orbit a predetermined number of times are taken from the loop orbit and reach the ion detector to be detected. Therefore, it is necessary to introduce ions into the loop orbit, and lead the ions out from the loop orbit.
In a mass spectrometer described in Patent Document 2, electrodes for deflecting ions are placed on the loop orbit. A voltage is applied to the electrodes when an ion passes through the electrodes, forming a deflection electric field which bends the orbit of an ion. Ions are accordingly led into or taken from the loop orbit. However, placing such electrodes on a loop orbit causes a decrease of the ions' transmittivity and possibly poses a decrease of analytical sensitivity. In addition, if the shape of the electrodes for deflection is simple such as a parallel-plate shape so as to simplify the structure, the convergence of the ions to be targeted is often adversely affected, resulting in a possible decrease of the mass resolution or the mass accuracy.
In the mass spectrometer described in Patent Document 1, an aperture for introducing ions or an aperture for leading ions out is placed on a portion of an electrode of a sector-formed electric field for forming a loop orbit. When an ion is introduced into or led out through the aperture, the voltage applied to the electrode is turned off (i.e. to zero potential). However, placing an aperture on an electrode for forming a sector-formed electric field causes disarrangement of the electric field near the aperture, which may adversely affect the turning of the ions. Hence, for practical purposes, a means of correction for correcting the disarrangement of the electric field is required. This leads to a complicated configuration.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-135060
Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-297267